There are numerous types of signalling devices presently available for rural type mailboxes to alert mail carriers that there is outgoing mail to be picked up by the mail carrier. For example, U.S. Patent Nos. 4,738,392, 4,728,028, 4,720,042, 4,711,391, 4,382,542, 4,365,740, and 4,344,559. Unfortunately, there are presently no similar types of signalling devices available for apartment mailboxes or apartment house mail receptacles. This presents a problem to apartment residents who have outgoing mail to be picked up by a mail carrier. Normally, such mail would be left by the resident in a common pick-up box or area near the apartment mailboxes. While this may be convenient for the resident and the mail carrier, it does not provide the resident with any security or assurance that such outgoing mail will actually be picked up by the mail carrier or that such outgoing mail will not be stolen prior to being picked up. If the resident attempts to leave outgoing mail inside of his or her individual locked mailbox, there is no way to indicate that this mail is to be picked up by the mail carrier and is not mail that had been previously delivered but that the resident had not picked up. Notes or messages left on the outside of an apartment mailbox indicating that there is outgoing mail inside are inconvenient and may be removed or torn down before being seen by the mail carrier. In addition, some apartment mailboxes are of a rear-loading type where themail carrier places the mail in the mailbox froma rear entrance. In these situations, the resident has no means of placing a note to be seen by the mail carrier because the resident does not have access to the rear entrance of the mailbox. Accordingly, there is a continuing need for a signal device to be used by apartment residents to alert mail carriers that there is outgoing mail in their mailboxes.